1. Field of the Invention
This invention relates to offset lithographic printing, and more particularly to apparatus for attachment to the down-stream end of an offset lithographic printing press, for coating purposes.
In offset lithographic printing, each printing stage includes a plate cylinder, to which the printing plates are fastened tightly around the circumference, the plate cylinder being equipped with superimposed inking, watering and wiping mechanisms. The plate cylinder does not come into contact with the paper to be printed, but transfers the image to an intermediate blanket cylinder, which has a specially composed smooth, rubber blanket surface. The blanket cylinder, having received the impression from the plate cylinder, in turn transfers it, or offsets it, onto the paper or other material, whilst it is being carried around an impression cylinder, located out of contact with the plate cylinder. Lithographic inks are oil-based, and special precautions normally need to be taken to dry the inks after their application to the printed material, as rapidly as possible without spoiling the quality of the printing, so that the printed material can be subsequently handled and stacked without damaging the applied printing.
2. Brief Description of the Prior Art
An alternative to conventional drying of printed sheets issuing from a lithographic printing press, is coating of the printed sheet with a water-borne system, to provide either a gloss or dull coating. Water-borne coatings, applied as an aqueous solution or emulsion, are capable of providing a simple protective barrier for the ink, which eliminates the need for the application of spray powder for drying purposes, and protects the ink from abuse whilst its normal setting and oxidation functions proceed. Much development with water soluble polymers has been undertaken in the last few years, to produce acceptable coatings for this purpose. When properly applied, the film is permeable and permits the passage therethrough of oxygen, to permit the normal setting and drying of the ink. In addition to this function of protecting printing inks after application to permit their proper drying, water-borne coatings can perform a useful decorative function to enhance the appearance of high quality, multi-colour printing work, for example phonograph record sleeves.
If the coating of printed material is conducted in a separate operation, after the material has been removed and isolated from the printing press, the operation is expensive and inconvenient, and does not contribute to the solution of the ink drying problems. It is known to apply water-borne and organic solvent lacquers in a separate operation from the printing, to provide special, decorative finishes. The use of solvent based lacquers introduces fire and explosion hazards.
There is a need in the industry for a simple and economical apparatus for application of water-based polymer coatings to printed material immediately after the printing thereof, i.e. in-line coating with the lithographic offset press.